The present invention relates to a method of and an apparatus for alleviating transmission shift shocks, particularly in an automatic transmission.
Automatic transmissions are now widely used in automotive vehicles as a drive for driving wheels of an automotive vehicle. When the automatic transmission shifts between reduction ratios or gear ratios, so-called shift shocks are caused mainly by the inertia of the engine because of a rapid and considerable change in a difference between an engine revolution speed and a transmission output shaft revolution speed as a result of shifting operation. This is described in detail referring to FIG. 6(a) where a 2-3 upshift operation is illustrated. As readily understood from FIG. 6(a), a change in the transmission output shaft revolution speed after the 2-3 shift is very small, whereas a change in the engine revolution speed is rapid and large. This causes a substantial increase in difference between the engine revolution speed and the transmission output shaft revolution speed after the upshift, causing a peak (T.sub.r1) in transmission output shaft torque to occur during shifting operation, resulting in substantial shocks.
In order to alleviate shift shocks, Laid-open Japanese Patent Application No. 58-77138 published May 10, 1983 discloses an apparatus for alleviating shift shocks wherein a transmission input shaft revolution speed (corresponding to an engine revolution speed) and a transmission output shaft revolution speed are detected, and control factors of an engine are controlled to increase or decrease output torque of the engine so as to keep an absolute difference between the transmission input and output revolution speeds below a predetermined value. In this control process, the detection of the transmission input and that of the output revolution speeds are repeated during shifting operation in order to enable torque up or torque down control in output torque of the engine which is carried out in response to the difference between the detected transmission input and output revolution speeds.
However, a delay occurs between an instant when the transmission input and output revolution speeds are detected and the subsequent instant when the engine torque control responsive to the result of the detection is carried out. This means that the torque control to be carried out based on a difference between the transmission input and output revolution speed detected at the preceding instant does not reflect an actual difference between the input and output revolution speeds taking place at the instance of the torque control. Therefore, the torque control carried out in this manner is not sufficiently accurate and thus does not result in satisfactory alleviation of shift shocks.
Besides, when the delay mentioned above results in a delay in converging the difference between the detected transmission input and output revolution speeds within the predetermined range, the torque control will continue even after the completion of the actual shifting operation. If this happens, a sluggish response to acceleration demand is encountered immediately after the shifting, resulting in poor acceleration performance.